This invention relates to a degradable container, especially a disposable degradable container for containing food or beverages. This invention also relates to a method of producing such a container.
Disposable food containers made of non-degradable materials have been available for a long time, and the negative effects of disposal of such food containers have on the environment have been widely recognized. Various alternative materials have been proposed for the production of degradable disposable food containers, and it is an object of the present invention to provide a new disposable degradable food container which is more environmentally friendly and to provide a new method of forming such a food container.
According to a first aspect of the present invention, there is provided a degradable container comprising from 60 wt % to 70 wt % of a plant fibre, from 10wt % to 30 wt % of an adhesive and from 0.1wt % to 5 wt % of a demoulding agent, wherein said adhesive is a modified urea-formaldehyde resin.
According to a second aspect of the present invention, there is provided a method of forming a degradable container, comprising the steps of (a) grinding a plant fibre to a size smaller than a pre-determined size; (b) mixing grounded plant fibre obtained under step (a) with a demoulding agent to form a premixed material; (c) mixing said premixed material with an adhesive into a powder form; (d) press-moulding said powder obtained under step (c) a first time under a pressure of 5-80 MPa; and (d) press-moulding said powder a second time under a pressure of 1.5-16 MPa into said container.
According to the present invention, plant fibre, at least one of the following: e.g. rice husk, corncob, peanut shell, coconut shell, wheat husk, bagasses, cereal stalks, corn stalks or sorghum stalks, was placed into a crushing machine, e.g. a dry turbo crushing mill traded by WDJ under the model No. WDJ-350, which has a yield of roughly 500 kg/hour, for grinding. The plant fibre so grounded was then fed into a sieving machine, which was a ZSX series tri-vibratory sieving machine of Model ZSX900-2S and of a yield of roughly 850 kg/hour. The sieving criterion was set at 40-mesh. Grounded plant fibre which could not pass through a 40-mesh sieve was passed back into the dry crushing mill for re-grinding until all the particulate could pass through a 40-mesh sieve. xe2x80x9cMeshxe2x80x9d (which may also be called xe2x80x9cporexe2x80x9d) is a scale for particulate material which represents the number of xe2x80x9cporesxe2x80x9d for every 25.4 mm (i.e. 1 inch) of a sieve. In this connection, the mesh (pore) number and the scale have the relationship as in Table 1 below:
Grounded plant fibre particulate which have passed through the sieving step were then mixed with the following ingredients:
(a) carboxymethyl cellulose (of a general chemical formula of (C6H9O4.OCH2COOH)n where n is a natural number) of up to 5 wt %, which acts as a viscosity increasing agent to increase the initial viscosity of the material;
(b) talc powder (Mg3(Si4O10)(OH)2) of up to 5 wt %, which acts as a flow aid to increase the fluidity of the material in the mould cavity during the moulding procedure;
(c) calcium stearate ([CH3(CH2)16COO]2Ca) of 0.1wt % to 5 wt %, which acts as a demoulding agent to promote the release of the resultant container from the mould cavity, and to smoothen the surface of the container;
(d) titanium dioxide (TiO2) of 0.5 wt % to 3 wt %, which acts as a whitening agent to improve the whiteness of the container;
(e) starch (of a general formula of (C6H10O5)n wherein n is a natural number) of up to 5 wt %, which acts as a modifying agent to improve the adhesive strength of the material, and also to increase the rate of degradation;
(f) polyvinyl butyral 
(where n is a natural number) of up to
5 wt %, which acts as a reinforcing agent to increase the tensile strength of the container, and to improve its other physical properties; and
(g) water (H2O) of up to 10 wt % to wet the ingredients and thereby to enhance stirring action, and to enhance the smoothness of the surface of the ultimate container.
The above ingredients and the grounded plant fibre were all fed into a rotating device for mixing at a speed of 500-1,300 r.p.m. (rounds per minute) for 2 to 5 minutes at a temperature of 30xc2x0 C. to 50xc2x0 C. Since heat was generated during the mixing procedure, no external heat was required to keep the temperature within the above range.
A modified urea-formaldehyde resin (to be further discussed below) was then added and the resultant solution was mixed by rotation at a speed of 2,500-3,600 r.p.m. for 10 to 25 minutes, and at a temperature of 50xc2x0 C. to 80xc2x0 C. The resultant material was then ready for use in the formation of the container. It may also be stored for future use. It is important to control the water content of the resultant material such that it falls within the range of 15 wt % to 22 wt %.
The resultant material obtained above was then introduced into a mould for subsequent moulding (thermo-setting). The material was firstly pressed at a temperature of 100xc2x0 C. to 200xc2x0 C. and at a pressure of 5 to 8 MPa for 5 to 10 seconds. The pressure was then reduced to normal atmospheric pressure for 5 to 30 seconds. Since the material contained water, during the thermo-pressing process, the water vaporized. Reduction of the pressure to atmospheric pressure allowed such water vapour to be exhausted in time, thus preventing air bubbles from being trapped in the container.
The product was then re-pressed for a period of 5 to 30 seconds at a pressure of 1.5 MPa to 16 MPa, to form the formal container. The mould was unloaded to allow the container to be retrieved therefrom. The mould was then cleaned for the next cycle of operation. The moulding procedure is in fact a curing process, under which the material was cured under pressure and temperature to form the container.
The basic criteria for the design of the mould are that the shape and dimension of the cavity of the mould should be identical to those of the container, the positioning of the mould should be reliable, the mould should be of sufficient strength, and the mould should have the necessary exhaust channel, and the necessary space for the overflow of surplus moulding material.
The container retrieved from the mould was then trimmed to cut off any unwanted parts, in order to standardize the shape of the product. The surface of the container was then coated with a film of water base terpolymer to enhance its resistance to heat and chemicals. The water base terpolymer is vinyl acetate-ethylene-acrylic acid copolymer, with cross-linking agent and silicone anti-foaming agent.
The vinyl acetate-ethylene-acrylic acid copolymer is of a general chemical formula of: 
where n, nxe2x80x2 and nxe2x80x2 are all natural numbers, and R and Rxe2x80x2 may each stand for H, alkyl or other substituting groups. The cross-linking agent is methyl-triethoxy silicane (CH3Si[OC2H6]3). The silicone anti-foaming agent is siloxane emulsion of the general chemical formula of: 
where n is a natural number.
The container was then dried in a stove for about 5 minutes, and subsequently packed in a sterilized environment. The room is a relatively sealed place subject to ultraviolet irradiation regularly to eliminate various bacteria in the air, in order to prevent pollution of the container during packaging.